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VCT3801A 参数 Datasheet PDF下载

VCT3801A图片预览
型号: VCT3801A
PDF下载: 下载PDF文件 查看货源
内容描述: 视频/控制/图文电视IC系列 [Video/Controller/Teletext IC Family]
分类和应用: 电视
文件页数/大小: 172 页 / 2219 K
品牌: MICRONAS [ MICRONAS ]
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VCT 38xxA  
ADVANCE INFORMATION  
2.4.2. Demodulator  
2.4.4. Frequency Demodulator  
The entire signal (which might still contain luma) is  
now quadrature-mixed to the baseband. The mixing  
frequency is equal to the subcarrier for PAL and NTSC,  
thus achieving the chroma demodulation. For SECAM,  
the mixing frequency is 4.286 MHz giving the quadra-  
ture baseband components of the FM modulated  
chroma. After the mixer, a low-pass filter selects the  
chroma components; a downsampling stage converts  
the color difference signals to a multiplexed half-rate  
data stream.  
The frequency demodulator for demodulating the  
SECAM signal is implemented as a CORDIC structure.  
It calculates the phase and magnitude of the quadra-  
ture components by coordinate rotation.  
The phase output of the CORDIC processor is differ-  
entiated to obtain the demodulated frequency. After  
the deemphasis filter, the Dr and Db signals are scaled  
to standard CrCb amplitudes and fed to the crossover  
switch.  
The subcarrier frequency in the demodulator is gener-  
ated by direct digital synthesis; therefore, substan-  
dards such as PAL 3.58 or NTSC 4.43 can also be  
demodulated.  
2.4.5. Burst Detection / Saturation Control  
In the PAL/NTSC-system the burst is the reference for  
the color signal. The phase and magnitude outputs of  
the CORDIC are gated with the color key and used for  
controlling the phase-locked-loop (APC) of the demod-  
ulator and the automatic color control (ACC) in PAL/  
NTSC.  
2.4.3. Chrominance Filter  
The demodulation is followed by a low-pass filter for  
the color difference signals for PAL/NTSC. SECAM  
requires a modified low-pass function with bell-filter  
characteristic. At the output of the low-pass filter, all  
luma information is eliminated.  
The ACC has a control range of +30...6 dB.  
Color saturation can be selected once for all color  
standards. In PAL/NTSC it is used as reference for the  
ACC. In SECAM the necessary gains are calculated  
automatically.  
The low-pass filters are calculated in time multiplex for  
the two color signals. Three bandwidth settings (nar-  
row, normal, broad) are available for each standard.  
For PAL/NTSC, a wide band chroma filter can be  
selected. This filter is intended for high bandwidth  
chroma signals, e.g. a non-standard wide bandwidth  
S-VHS signal.  
For SECAM decoding, the frequency of the burst is  
measured. Thus, the current chroma carrier frequency  
can be identified and is used to control the SECAM  
processing. The burst measurements also control the  
color killer operation; they are used for automatic stan-  
dard detection as well.  
2.4.6. Color Killer Operation  
The color killer uses the burst-phase/burst-frequency  
measurement to identify a PAL/NTSC or SECAM color  
signal. For PAL/NTSC, the color is switched off (killed)  
as long as the color subcarrier PLL is not locked. For  
SECAM, the killer is controlled by the toggle of the  
burst frequency. The burst amplitude measurement is  
used to switch-off the color if the burst amplitude is  
below a programmable threshold. Thus, color will be  
killed for very noisy signals. The color amplitude killer  
has a programmable hysteresis.  
PAL/NTSC  
SECAM  
Fig. 2–5: Frequency response of chroma filters  
14  
Micronas  
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